The invention relates to the field of materials science. It relates to a coating which contains large volumetric quantities, preferably in the range from 20 to 90% by volume, of NiAl-xcex2 phase in a xcex3 matrix.
A large number of alloys which are used to coat gas turbine components, for example, are known. The gas turbine components, for example, turbine blades, are exposed to high temperatures and are to be protected from oxidation and corrosion by means of the coatings.
In order to fully exploit the advantage of a high temperature in order to increase the efficiency of the turbine and the excellent mechanical properties of the base material (for example single crystals or directionally solidified microstructures), it is necessary for the coating material not only to protect the base material against oxidation and corrosion but also not to impair the mechanical properties of the base material. In particular, a low ductile brittle transition temperature (DBTT) and consequently a certain ductility at low temperatures for the coating material are to be achieved.
Unfortunately, this is not the case with known coatings.
For example, U.S. Pat. No. 5,043,138 describes a coating which is a typical Ni-base superalloy (single crystal alloy) with the addition of yttrium and silicon. Although these elements improve the creep rupture strength and, moreover, lead to a low ductile brittle transition temperature, the other elements which it contains, namely W, Mo and the small amounts of Cr and Co have an adverse effect on the resistance to oxidation.
Although the high-strength NiAl alloys which have been developed in recent years are in certain ways able to compete with the Ni-base superalloys, they have the drawbacks of a low toughness compared to ductile, high-toughness Ni-base superalloys and a high DBT temperature (R. Dariola: NiAl for Turbine Airfoil Application, Structural Intermetallics, The Minerals, Metals and Materials Society, 1993, pp. 495-504), which is reflected by a low ductility of these alloys at low temperatures. The xcex2 phase of the NiAl alloys has an ordered cubic B2 crystal structure (CsCI Prototype) and comprises two simple cubic cells which penetrate one another and in which the Al atoms occupy the cube corners of one sublattice and the Ni atoms occupy the cube corners of the other sublattice. The xcex2 phase is coarse and therefore brittle.
U.S. Pat. No. 5,116,438 has disclosed xcex2-phase Ni aluminides which are microalloyed with gallium. With about 0.25 atom % Ga, they exhibit a significant improvement in the ductility at room temperature. A higher Ga content has adverse effects.
The addition of small quantities of boron, and Hf, Zr, Fe and combinations of these elements, to Ni3Al alloys in order to improve the ductility is known, for example, from U.S. Pat. No. 4,478,791 and U.S. Pat. No. 4,612,165.
The invention improves the ductility of NiAl coatings which have a high content of xcex2 phase in a xcex3 matrix. The xcex2 phase may have various compositions, for example NiAl, NiAlCr, NiAlMo, NiAlTi.
According to one aspect of the invention, this is achieved by the fact that the coating containing NiAl-xcex2 phase contains the following microalloying elements (data in % by weight) 0.1-8 Fe and/or 0.1-8 Mo and/or 0.1-8 Ga, where the total Fe, Mo and Ga content is at most 10%.
Advantages of the invention are that the ductility of the coating is significantly improved. The microalloying with Fe, Ga and Mo results in the xcex2 phase becoming finer and consequently in the ductility increasing, without the resistance to oxidation being reduced. If the ranges indicated are exceeded, there will be unfavorable consequences for the ductility and the resistance to oxidation and corrosion.
It is particularly expedient if the coating contains max. 4% by weight Fe, Ga, Mo.
Furthermore, it is advantageous if, in addition, small amounts of B; (0.0005-0.9, preferably 0.001-0.5% by weight), Zr (0.0005-1.0, preferably 0.001-0.5% by weight), and/or C; (0.0005-0.8, preferably max. 0.5% by weight) are added. B, Zr and C; strengthen the grain boundaries and the xcex2/xcex3 phase boundaries.